Most chess computers with "presence sensor" systems use reed switches and magnets to track the moves of the chess pieces on the chessboard. These systems usually have one reed switch placed under each square of the chessboard (64 reed switches in total). A magnet is positioned in the base of each chess piece. When a piece is placed on a chess square, the reed switch is activated and remains closed until the piece is removed. Thus, the identities of the pieces on the game board may be tracked by the electronics if the pieces start from predefined positions, e.g. a new game or a set-up position defined.
The polarity of the magnets in the pieces is randomly oriented, i.e. some pieces have a south pole downwardly while others may have the north pole facing downwardly. Both will activate the reed switches in normal non-capturing moves where a piece is lifted up and placed onto the new destination square. The electronics can sense this open and close circuit of the reed switch with no problem.
The situation is different if it is a capturing move. On the square of the piece to be captured, the reed switch is closed because there is a piece on it. The piece is then lifted and removed, the reed switch becoming open circuit, and then the capturing piece is put onto the square whereupon the reed switch closes again. The close-open-close action of the reed switch informs the electronics of the capturing move.
If the capturing action is very fast, however, the reed switch may have no opportunity to open since the magnetic force of a capturing piece with the same polarity may overlap and mutually strengthen the magnetic field and so activate the reed switch before the captured piece is removed; thus the reed switch may never open and the electronics will then not register the final step of the move.
With chess, it is not unusual for many good chess players to capture a piece by seizing it between the thumb and index finger of a hand, replacing it by the capturing piece held between the index and middle finger of the same hand or vice versa. Therefore this problem is particularly acute with chess.
Thus, error may occur if the capturing action is fast and if the piece to be captured and the capturing piece have magnets of the same polarity. However, if according to the invention the polarities of the magnets are intentionally oriented so that all of black pieces, i.e. pieces of one set, have their magnets with the same polarity facing downwards and all of the white pieces, i.e. the pieces of the other set, have the opposite, then a close-open-close action on the reed switch of the respective square or board position can be guaranteed. Thus, if a reed switch is, for example, activated by a magnetic field of a north pole, replacing it with a magnetic field of the opposite polarity will cause the reed switch to open as the null-field zone approaches it. The reed switch will close again finally once the capturing piece controls the square.
From experimental results, the duration of opening of the reed switch in these circumstances is at least 4 milli-seconds even when the capturing action is reasonably fast, compared with a typical bounce time of a typical reed switch of 0.4 milli-second, i.e. an order of magnitude smaller. Thus, this is enough time for the electronics and/or the controlling software to distinguish capture from contact bounces of the reed switches.